Generally, a crank shaft 22 for engine is, as shown in FIG. 4, rotatably supported at its journal portions 22a by bearings 23 provided in a crank case 21 of engine 20. Pins 22b are connected with connecting 24 in a manner of being freely rotatable, so that a force applied to the crank shaft 22 tends to cause the crank shaft 22 to be bent around the bearings 23 when a piston 26 is moved down due to an explosion in a combustion chamber 25. Therefore, the journal portions 22a in association with bearing metals 27 of the bearings 23 are required to have high durability.
Hence, specific kinds of steel materials having high durability are selected for the crank shaft 22 to improve durability at its journal portions 22a. This, however, leads to problems such as an increase in the cost of materials and in weight.
The present invention has been designed to overcome the above problems. An object of the present invention is to improve the durability of the journal portions of the motor rotary shaft without an increase in the cost of materials and in weight.